The present invention relates generally to mineral separation and more particularly to the control and automation of hydraulic classifiers of the general type shown in U.S. Pat. Nos. 2,696,298, 2,681,751 and 3,412,858, for efficiently separating rock from raw coal prior to delivery to cleaning devices such as Coal-washing tables. 2. Description of the Prior Art
The aforesaid patents relate to hydraulic classifiers wherein the spigot discharge from the classifier constitutes the feed to an ore-concentrating or coal-washing table, both of which require a reasonably constant rate of feed in order to function in a desired, optimum manner.
Under conditions currently prevailing in the coal industry wherein poorer grades and thinner seams of coal are being mined, the percentage of shale, rock and reject material contained in the run-of-the-mine coal is markedly increased in many instances. The higher percentage of rejects in total plant feed and particularly in the smaller size fractions that would be cleaned on coal-washing tables or similar cleaning devices, limits and reduces the overall capacity of the equipment and the plant as a whole. The ultimate capacity is determined either by the quantity of reject material that the equipment can separate and discharge as a separate product or by rated feed capacity of the equipment, whichever occurs first. Thus, a plant with a rated feed capacity of 250 tons per hour or 621/2 tons per hour of rejects, such rejects constituting 25% of the feed, would function satisfactorily at the 250 tons per hour rate. If, however, the percentage of rejects is increased to 30%, plant feed would have to be reduced to 208 tons per hour; at 40%, it would need to be reduced to only 156 tons per hour and at 50% to only 125 tons per hour. However, if the percentages of rejects is only 20% or less, the plant capacity would still be only 250 tons per hour.
It is therefore evident that plant capacities can be enhanced and/or maintained at normal feed rates when handling feeds containing excess rejects, if a portion of such excess could be inexpensively stripped out of the raw coal feed ahead of the tables. This can be best done by hydraulic classification but not with classifiers as described in the aforementioned patents.
In these prior art classifiers, the incoming feed after passing over the first cell goes on to the following cells in series where each classifies and discharges succeedingly smaller size particles according to their settling rates in water. Only the first cell, or in some rare cases the first and second cells, can separate coarse size, high gravity refuse material from desirable coal as those familiar with the classification art are well aware. Thereafter, each succeeding cell will discharge as its spigot product, a mixture of particles composed of coarser sizes of light gravity material and relatively finer sizes of higher gravity particles having exactly the same settling rates in water. The varients in the size range of particles in any spigot will be a function of the differential in specific gravity between the materials.
Thus, the first cell is the only cell of value in stripping out refuse to obtain a clean product. However, since there is a mechanical limitation on the diameter of the sorting column, and hence on the size of the cell itself, as disclosed in U.S. Pat. No. 2,696,298, and since any sizeable plant would thus require more than the single cell unit to handle required capacity, multiple units having a series of cells would be indicated rather than installing a battery of single cells in parallel in accordance with the present invention. In a unit of this invention wherein the multiple cells are in parallel, the launder section above the cells is so designed as to permit transverse flow with each feed stream passing over one cell only, instead of one larger stream flowing lengthwise over a plurality of cells ranged in series.
Since the spigot discharge of each cell is not required to be treated on a table and can go directly to waste, the discharge from the cell can be intermittent and the elaborate and costly controls used on the prior art, series type, high capacity classifiers will not be required.